This invention relates to a laser for providing a single longitudinal mode (SLM) output.
A laser resonator can sustain waves which have any integral number of half wavelengths. These discrete frequencies of oscillation are known as modes of the resonator. In principle all mode frequencies which lie within the gain bandwidth of the laser medium can oscillate. Usually lasers run on many modes simultaneously, they are multi-transverse and multi-longitudinal modes. Each mode starts from noise (spontaneous emission), a random thermal like process. Initially the mode intensities grow exponentially with time and each mode competes with its neighbours for the available gain. This fierce competition leads to the suppression of weak modes, those that see low net gain or which started with low intensity. The modes nearest the gain centre see the highest gain and reach the saturating intensity first. This line narrowing process can be enhanced by lengthening the build-up time of the oscillation in order to allow more time for mode competition. Etalons and other interferometric techniques are also used to introduce additional losses on unwanted modes. However getting a high gain solid state laser, such as Neodymium: YAG or YLF, to run SLM is still difficult and it has previously been found necessary to ensure the oscillator also runs in a single transverse mode.
The present inventors have established that a diode pumped end pumped Neodymium master oscillator will run TEMoo and SLM. This is discussed in the paper: Single Frequency, end pumped Nd:YLF laser excited by a 12 mJ diode-laser array: Optical Letters; Dec. 1, 1992, Vol. 17, No. 23, the contents of which are hereby incorporated by way of reference. The SLM performance is achieved by slow Q-switching and cavity length control and by pumping the oscillator at low level (1 mJ output). The single mode which oscillates is generally that which is closest to the centre of the gain bandwidth (line centre). However this mode has been found to be unstable as the environment changes, and the mode frequency drifts away from line centre until two longitudinal modes occur.
An extension to the technique of "pre-lase" Q-switching which ensures that a TEMoo mode, Q-switched NdYAG laser produces SLM oscillation on every shot is disclosed in Optics Communications, 43 (1982) 6, pages 414-418. This discloses monitoring the pre-lase output and only opening the Q-switch when SLM oscillation is detected, relying on the heat generated in the lasing medium in conjunction with the coefficient of expansion of the lasing medium to perform a resonator length sweep with a sufficient shift in path length to ensure that SLM oscillation is present at some point during the sweep, normally within the period in which four or five pre-lase pulses occur. The document also discloses including an actuator to physically alter the path length of the resonator if the coefficient of expansion of the laser medium is not sufficient.